This invention relates to the repair of front seals in a waterwall of a boiler furnace. Typically, such furnaces are designed to include front seal boxes where, for example, superheater and reheater pipes pass through the furnace waterwall and into a header enclosure which contains a superheater outlet header and a reheater outlet header. As these seal boxes are cycled thermally during use, the original seals can give way to the point that they permit ash leakage into the header enclosure.
Ash leakage into a superheater header and reheater header enclosure is of concern since at the high temperatures resulting from the burning of fuel in the furnace, fractions of the ash become partially fused and sticky thereby adhering to surfaces and permitting chemicals in the ash to attack materials such as the alloys used in superheaters and reheaters. In addition, the buildup of ash in the bottom of the enclosure creates a serious weight problem which can dimensionally distort and damage the enclosure and the pipes and headers it contains and their connections to the system. An extreme buildup in weight within the elevated header enclosures also creates an obvious safety problem.
As will be well recognized by those skilled in the art, the function of a superheater is to increase the temperature of the steam generated in the boiler. Steam enters the superheater at saturated temperature in a practically dry saturated condition, and consequently the absorption of heat appears as sensible heat in increasing the steam temperature.
The reheater receives superheated steam which has partly expanded through the power generating turbine. The function of the reheater is to re-superheat this steam to a desired temperature.
Furnace design must take into consideration water heating and steam generation in the waterwalls as well as the processes of combustion and ash production. Practically all large modern boilers are designed with walls comprised of water cooled tubes to form complete metal coverage of the furnace enclosure. Waterwalls usually consist of substantially vertical tubes arranged tangent, or approximately so, and these are connected at top and bottom to headers. In addition, areas outside of the furnace which form enclosures for sections of superheaters, reheaters and often economizers are also designed in a manner similar to the furnace, using either water or steam cooled tube surfaces. Present practice is to use tube arrangements and configurations which permit practically complete elimination of refractories in the areas exposed to high temperature gases. Accordingly, it is important to have effective seals against ash leakage where pipes for the boiler components must pass into and out of the furnace through waterwalls.